This invention relates to optics, and it is concerned more specifically with detection of, measuring of and correction for movement of an optical target during a procedure involving the optics, such as a medical or industrial procedure involving a laser beam focused on the target.
In laser delivery systems, and particularly in systems for delivering a surgical laser beam toward target tissue which is being operated upon, it is important either to have the target totally immobilized during the procedure, or to quickly follow or "track" optically the target movements occurring. These movements can, in a surgical setting, be caused by the patient's being unable or unwilling to sufficiently control voluntary musculature, and/or by operation of involuntary musculature (e.g. heartbeat, breathing). This is true for imaging systems which present to a user images or data relating to the configuration or topography of the target and/or relating to the location of a laser beam's focus, when fired, on or in the target. It also is true for the focussing system itself.
The principle of confocal microscopy is well known. The principle involves the focussing of an optical system on an object or position in front of an objective lens, with a second focal point being located at a pinhole in the system behind the objective lens. If the depth of origin of specularly reflected light entering the system through the objective lens changes, the intensity of light at an image plane behind the pinhole becomes less due to shifting of the second focal point or beam waist away from the pinhole.
Bille U.S. Pat. No. 4,881,808 disclosed an imaging system for determining the location of an object such as the cornea of the human eye. Bille's disclosed system utilized the principles of confocal microscopy in determining the location of a series of points on the cornea, in order to draw a picture electronically of the corneal shape and thus to define its position In determining the location of each point, Bille moved the pinhole in a confocal microscope system in order to find the pinhole location wherein light was focussed through the pinhole, i.e. the maximum light intensity on a photodetector behind the pinhole. Each point taken in Bille's system gave a relative depth for a particular aiming location of the imaging system. With a number of such points taken, each at a different aiming location, Bille's system was able to obtain coordinates of a series of points thereby drawing a picture of the location of the curved shape being imaged. The system of the Bille patent therefore operated in a manner similar to conventional confocal microscopy, in that transverse shifting of point locations investigated resulted in a collection of measurements which could be used to generate topographical mapping of the object being imaged.
Bille's system differed from the present invention principally in that Bille's purpose and objective were to image a shape such as an ocular cornea, and to determine the location of that shape. The system of the present invention is not an imaging system but rather a system for detecting movements of a target (such as an ocular cornea) during a procedure such as laser ophthalmic surgery. The system of the invention has the objective of monitoring the depth of a point of specular reflection along a single optical axis line, not imaging the shape of the cornea or locating a series of points in space.
Further, a principal purpose of the invention is to correct for and follow depth movements of a target such as the cornea of an eye during a surgical procedure; this can be accomplished by driving an objective lens (or other optics) of the system in response to the changes in intensity of light imaged on a photodetector behind a pinhole caused by changes in the depth position of the specular reflection point on the target.